1. Field of the Invention
The present invention relates to a polarization detector used for optical pickup and the like, and more particularly to a polarization detector equipped with a polarization diffraction element for separating an incident light into two light beams with different polarization components.
2. Description of the Related Art
FIG. 8 shows a cross-sectional view of a polarization diffraction element 100. The polarization diffraction element 100 includes a transparent substrate 101 made of glass or the like, on one surface of which a diffraction grating 102 represented by hatching is formed. The grating pitch of the diffraction grating 102 is formed so as to nearly correspond to the wavelength of the light to be used. For example, the diffraction grating 102 is formed of a photoresist, having a thickness of 1 .mu.m and a grating pitch of 0.5 .mu.m. The diffraction grating 102 is formed by a two-beam interference method or the like.
When the grating pitch of the diffraction grating 102 is formed so as to be nearly equal to a light wavelength, as described above, the following polarization characteristics are known to be obtained. (See K. Yokomori, "Dielectric Surface-Relief Gratings with High Diffraction Efficiency", Applied Optics Vol. 23, No. 14, pp. 2303, 1984.)
As shown by the arrow in FIG. 8, the diffraction grating 102 of the polarization diffraction element 100 having the above-mentioned structure allows a P-polarized light L.sub.P to pass therethrough at nearly 100% and an S-polarized light L.sub.S to diffract at nearly 100%. The P-polarized light L.sub.P has an electrical field which vibrates in the direction parallel the drawing sheet of FIG. 8. The S-polarized light L.sub.S has an electrical field which vibrates in the normal direction with respect to the drawing sheet surface.
When light L, for example, having a wavelength of 0.8 .mu.m is incident upon the above-mentioned polarization diffraction element 100, a majority of the P-polarized light L.sub.P passes through the diffraction grating 102 as a zeroth-order diffracted light L.sub.a, with a minority being diffracted as a first-order diffracted light L.sub.b. On the other hand, a majority of the S-polarized light L.sub.S is diffracted by the diffraction grating 102 as a first-order diffracted light L.sub.b, with a minority passing through the diffraction grating 102 as a zeroth-order diffracted light L.sub.a.
As a polarization diffraction element used for optical pickup of a magneto-optical element utilizing the above-mentioned polarization characteristics, Japanese Laid-Open Patent Publication No. 2-259702 by the present inventors discloses a polarization diffraction element, a polarization detector, etc. These polarization diffraction elements have a structure formed by considering the influence of wavelength fluctuation of an incident light, the configuration in the case where a polarization detector including a photodetector is constituted, the improvement of the degree of separation of P- and S-polarized lights.
FIG. 9 shows an example of the abovementioned polarization diffraction element. A polarization diffraction element 1.10 includes a transparent flat substrate 111 made of glass or the like. On both surfaces of the substrate 111, a first diffraction grating 112 and a second diffraction grating 113 are formed, respectively. A grating pitch D.sub.1 of the first diffraction grating 112 and a grating pitch D.sub.2 of the second diffraction grating 113 are set so as to nearly equal the wavelength .lambda. of the incident light L. In addition, the groove direction of the first and second diffraction gratings 112 and 113 corresponds to the direction of a normal line with respect to the drawing sheet surface of FIG. 9.
FIG. 10 is a cross-sectional view of the first and second diffraction gratings 112 and 113. As shown in this figure, the first diffraction grating 112 has a plurality of sinusoidal convex portions 112a with the grating pitch D.sub.1, each grating pitch D.sub.1 being the same, further the second diffraction grating 113 has a plurality of sinusoidal convex portions 113a with the grating pitch D.sub.2, each grating pitch D.sub.2 being the same. In this case, the grating pitch D.sub.2 is set so as to be slightly larger than the grating pitch D.sub.1. The first diffraction grating 112 and the second diffraction grating 113 can be formed as a relief type diffraction grating by, for example, etching the substrate 111.
The operation of the polarization diffraction element 110 with the above-mentioned structure will be described.
The first diffraction grating 112 and the second diffraction grating 113 allow the P-polarized light L.sub.p of the incident light L to pass therethrough at nearly 100%, and allow the S-polarized light L.sub.S to diffract at nearly 100%. Since the grating pitch D.sub.2 is larger than the grating pitch D.sub.1, in the case where the incident light L with a predetermined wavelength is incident upon the polarization diffraction element 110 through the first diffraction grating 112, the following is caused: As shown by an arrow in FIG. 9, a transmitted light L.sub.c transmitted through the first and second diffraction gratings 112 and 113 and a diffracted light L.sub.d diffracted by the first and second diffraction gratings 112 and 113 output from the polarization diffraction element 110 at an angle difference of angle .alpha. therebetween.
At this time, the incident light L is incident upon the first diffraction grating 112 at an incident angle .theta..sub.1, and the S-polarized light L.sub.S of the incident light L is diffracted in a direction so as to form an angle .theta..sub.2 with respect to a normal line n of the polarization diffraction element 110. In addition, the S-polarized light L.sub.S is diffracted in a direction so as to form an angle .theta..sub.3 with respect to a normal line n and output from the polarization diffraction element 110. In this case, the difference angle, angle .alpha., is represented by .theta..sub.1 -.theta..sub.3. Moreover, the angles .theta..sub.1, .theta..sub.2, and .theta..sub.3 satisfy the following Formula (1): EQU Sin.theta..sub.1 +Sin.theta..sub.2 .lambda./D.sub.1 EQU Sin.theta..sub.2 +Sin.theta..sub.3 =.lambda./D.sub.2 ( 1)
where .lambda. is an oscillation wavelength of the incident light.
FIG. 11 is a front view schematically showing a polarization detector equipped with the polarization diffraction element 110 which functions as described above. The polarization detector includes the polarization diffraction element 110, a converging lens 120 for converging light which output from the polarization diffraction element 110, and a pair of photodetectors 130a and 130b for detecting the intensity of light converged by the converging lens 120. The pair of photodetectors 130a and 130b are provided in one package 130.
In the polarization detector with the abovementioned structure, a data signal is given to the polarization diffraction element 110 as the incident light L, and the incident light L is separated into the P-polarized light Lp and the S-polarized light l.sub.S by the polarization diffraction element 110, as described above, output from the polarization diffraction element 110. The outgoing light is converged on the photodetectors 130a and 130b by the converging lens 120, whereby the optical signal is converted into an electrical signal by the photodetectors 130a and 130b.
In the above-mentioned polarization diffraction element 110, the difference between the grating pitch D.sub.1 of the first diffraction grating 112 and the grating pitch D.sub.2 of the second diffraction grating 113 is very small. Thus, the fluctuation of the difference of angle .alpha. caused by the wavelength fluctuation of the incident light L as shown in FIG. 9 is small and there is an effect that a positional shift of beam spots converged on the photodetectors 130a and 130b can be prevented.
A small-sized and light-weight optical pickup device has been realized by incorporating the polarization detector into the optical pickup.
However, in the above-mentioned polarization detector, a reflected light generated in the polarization diffraction element 110 and the incident light L cause interference. Because of this, the intensity of light from the polarization diffraction element 110 is fluctuated and a noise is caused in the signal reproduced by the photodetectors 130a and 130b, deteriorating the quality of the signal.
This problem will be described in detail as follows:
FIG. 12A is a cross-sectional view showing the polarization diffraction element 110. FIG. 12B shows the shape of beam spots on the first diffraction grating 112. As shown in FIG. 12A, the S-polarized light L.sub.S of the incident light L with respect to the polarization diffraction element 110 is diffracted by the first diffraction grating 112 to become a first-order diffracted light L.sub.1. Moreover, the first-order diffracted light L.sub.1 is separated into an outgoing light L.sub.3 and a reflected light L.sub.2. The outgoing. light L.sub.3 is light which undergoes the first order diffraction by the second diffraction grating 113. The reflected light L.sub.2 is light which is reflected from the second diffraction grating 113. Because of this, as shown in FIG. 12B, two beam spots of the incident light L (mainly, the S-polarized light L.sub.S) and the reflected light L.sub.2 are formed, and the light which corresponds to an overlapped portion of both of the beam spots is an interference light L.sub.i. As shown in FIG. 12A, among the interference light L.sub.i, a light L.sub.i1 reflected from the first diffraction grating 112 undergoes first-order diffraction by the second diffraction grating 113 to become a light L.sub.i2, and the light L.sub.i2 is irradiated to the photodetector 130b (not shown in FIG. 12A) together with the original S-polarized light L.sub.S.
As described above, the intensity of the interference light L.sub.i proceeding to the photodetector 130b is determined by the phase difference between the reflected light L.sub.2 from the first diffraction grating 112 and the S-polarized light L.sub.S of the incident light L. The phase difference depends on an oscillation wavelength of a semiconductor laser which is the light source of the incident light L. However, since the oscillation wavelength of the laser fluctuates, the phase difference also fluctuates, resulting in the fluctuation of the intensity of the interference light L.sub.i. The fluctuation of the light intensity causes a noise in the data signal transmitted through the polarization diffraction grating 110, deteriorating the quality of the signal. The reason for the fluctuation of the semiconductor laser is as follows: The semiconductor laser is weak to backward light, so that in the case where the semiconductor laser is used for optical pickup, a laser driving current is overlapped with a high frequency current for the purpose of suppressing the backward light.
In the case of the P-polarized light L.sub.p of the incident light L, assuming that the incident angle of the zeroth-order diffraction light with respect to the second diffraction grating 113 is about 35.degree., a reflected light is hardly generated as shown in FIG. 13. Thus, the above-mentioned problems do not arise.